There exists a desire for high-viscosity organopolysiloxanes which have a branched structure and are used in products such as cosmetics, personal care compositions, home care compositions, mold release agents, slip agents, coating agents, textile finishes and resin modifiers to be rendered into emulsions having a small particle size and a good stability over time. However, when a high-viscosity organopolysiloxane having a branched structure is directly emulsified, the lower limit in the size of the emulsion particles is about several microns; achieving a smaller particle size is difficult, and the resulting emulsion has a poor stability over time. Accordingly, various methods for producing emulsions of high-viscosity organopolysiloxanes having a branched structure by emulsion polymerization have been investigated in order to obtain emulsion particles with a good stability over time.
For example, methods for carrying out the emulsion polymerization of a cyclic siloxane oligomer and a trialkoxysilane in an emulsified state by using a strong acid or strong alkalinity are known (Patent Document 1: JP-B S56-038609; Patent Document 2: JP-A S63-286434). Using these methods, it is possible to obtain an emulsion having an emulsion particle size of 300 nm or less.
In recent years, there has come to be a desire for products in which the content of octamethylcyclotetrasiloxane is suppressed. In the methods described in Patent Documents 1 and 2, the organopolysiloxane included in the resulting emulsion is known to contain at least 40,000 ppm of octamethylcyclotetrasiloxane, and so methods for reducing this content are being investigated.
For example, methods are known wherein an organopolysiloxane which has a viscosity at 25° C. of from 3,000 to 100,000 mm2/s and an octamethylcyclotetrasiloxane content of up to 1,000 ppm and which is capped at the ends of the molecular chain with silanol groups is emulsified, following which emulsion polymerization is carried out at a temperature below 40° C. and in the presence of an acid catalyst (Patent Document 3: JP No. 5382273). It is claimed that by using such a method, an emulsion in which the amount of octamethylcyclotetrasiloxane included in the organopolysiloxane is up to 3,000 ppm can be obtained. Mention is also made that by adding a trialkoxysilane to this emulsion, it is possible to introduce branched units onto the resulting organopolysiloxane chain. However, there exists the drawback that, even when a trialkoxysilane is reacted with a high-viscosity organopolysiloxane, due to a difference in reactivity between the organopolysiloxane capped at the ends of the molecule chain with silanol groups and the trialkoxysilane, branched units are not uniformly incorporated into the siloxane chain and so the viscosity of the organopolysiloxane chains that form does not increase.
Hence, there is a need to establish a method for producing an emulsion composition of a high-viscosity organopolysiloxane having a branched structure, which composition has a small particle size and a good stability over time, and which method suppresses the formation of octamethylcyclotetrasiloxane included in the organopolysiloxane as a by-product.